Its better to have the 2 EX4300 under Virtual Chassis environment and make it logically behaving like one for the Cisco access switch.
If you start your environment with MSTP and in future if you need to have more VLANs, you need to add more MSTP instance or re-configure the existing. I would request you to put your effort in make the device as VC and start testing from that.
If you still need to test this scenario, i can lab it up with Juniper as Distribution and Access switch and come to you with solution, because STP with same brand works as expected, with different brand, it will require lots of trail and error as well as research.
Go with Virtual Chassis 
Thanks
Arul